The human body displays cyclic patterns of gene expression, hormone secretion, and behavioral activity reflecting the 24-hour light/dark cycle. Recent studies reveal that the PAS domain transcription factors expressed within the suprachiasmic nucleus of the hypothalamus play a critical role in establishing and regulating the body's central circadian clock. However, new evidence indicates that tissues peripheral to the central nervous system contain their own, independent circadian clocks. Clues in the literature suggest that a peripheral, "osteogenic" circadian clock exists; serum levels of osteocalcin, alkaline phosphatase, and C- telopeptide exhibit a reproducible, oscillatory diurnal pattern in human subjects and animal models. This R21 proposal seeks to confirm the existence of a "peripheral" circadian clock within bone tissue by testing the following hypotheses: (Hypothesis 1) That a peripheral clock can be found within osteogenic tissues and that members of the "clock" PAS protein family are the mechanical regulators of its timing. Aim 1 will use real time PCR to document the temporal expression profile of circadian rhythm genes in murine bone. Cohorts of young (4 wk) and older (8 wk) mice will be entrained to a 12 hr light12 hr dark cycle. Cortical and intramembranous bones will be harvested for total RNA from groups of mice at different times around the clock. The expression profile of circadian rhythm genes will be determined by quantitative real time PCR. (Hypothesis 2) That bone-specific genes can demonstrate a circadian pattern of expression. Aim 2 will examine the expression profile of the osteocalcin promoter with respect to circadian mechanisms. Goal 1 in vitro studies will co-transfect expression vectors for the circadian rhythm genes with an osteocalcin promoter/luciferase reporter construct. Luciferase activity will be determined in transfection combinations to determine if the circadian rhythm proteins enhance or inhibit expression from the osteocalcin promoter. Goal 2 in vivo studies will use mice transgenic for the osteocalcin promoter/luciferase reporter construct. Luciferin induced light emission from the intact skeleton of individual mice will be visualized at different times of day using a bioimaging chamber. Findings from this innovative proposal have clinical implications regarding the optimal timing for daily medication regimens in bone diseases and for elective orthopedic and orthodontic procedures.